MARYINKA, Ukraine — Artillery fire killed at least four people in an overnight attack on a residential area in eastern Ukraine, spurring more people to flee the besieged city of Donetsk and its suburbs on Saturday to take their chances elsewhere.

Pro-Russian insurgents last week retreated from the strategic city of Slovyansk and holed up in Donetsk, a city of one million, and potentially the final frontier for the rebels. The overnight artillery strike in Maryinka, a western suburb of Donetsk, hit four apartment blocks near a rebel base. It was unclear, however, which side fired at the buildings.

“Even the fascists didn’t do what they did. So many peaceful citizens,” said a resident who gave her name as Valentina Mikhailovna. “Look what’s happening! Dead, dead, dead people!”

One damaged apartment was still burning at midday despite the efforts of firefighters. Another building was smouldering.

AP journalists heard outgoing mortar fire from the area at noon on Saturday, suggesting that the rebels sometimes fire at the troops from the residential area, prompting Ukrainian forces to return fire.

Fighting between the rebels and government troops has left more than 400 dead and tens of thousands have fled their homes, in many cases crossing the border into Russia.

The rhetoric on both sides is increasingly intransigent.

The attack in Donetsk came hours after Ukraine’s president, Petro Poroshenko, vowed vengeance for the deaths of 19 troops in an insurgent rocket attack near the Russian border.

“For every life of our soldiers, the militants will pay with tens and hundreds of their own,” Poroshenko said. “Not one terrorist will evade responsibility. Everybody will get what is coming to them.”

Meanwhile, the European Union moved on Saturday to impose sanctions on 11 leaders of the pro-Moscow rebellion.

Targets of the asset freeze and travel ban included two Russian spin doctors, Alexander Borodai, prime minister of the separatist Donetsk People’s Republic, and his counterpart in the Luhansk People’s Republic, Marat Bashirov.

Both formerly worked for Russian tycoons.

The names published in the EU’s Official Journal brought the total of people subject to the bloc’s Ukraine-related sanctions to 72, as well as two companies whose EU-based assets have been ordered frozen.

The sanctions, agreed by EU ambassadors on Wednesday, go into effect Saturday.

A Russian diplomat chided Brussels for the move.

Vladimir Chizhov, the Russian envoy to the EU, said in comments carried by Interfax on Saturday that Moscow believes in bringing peace to eastern Ukraine through negotiations, and that “blacklisting opponents isn’t helping to build a dialogue with them.”

Ukraine has refused to negotiate with rebels whom it describes as terrorists, although a Kyiv envoy has taken part in a round table with Borodai along with representatives of Russia and the Organization for Security and Cooperation in Europe.

Rebels have rejected Ukraine’s call to lay down their arms as a condition for cease-fire negotiations.

Nataliya Vasilyeva in Moscow and John-Thor Dahlburg in Brussels contributed to this report.